ABSTRACT

Ion sensitive electrode-based meters appear to have potential as diagnostic tools for nutrient monitoring through the use of petiole sap extracts. The rapid turnaround time, and sound scientific basis for the technology has simulated interest in the use of these meters for N and K management of cotton. Potassium and NO₃-N meters were evaluated in replicated N and K rate studies to determine if petiole sap extracts (obtained with a garlic press) were responsive to fertilizer treatments. Sap extracts were compared with conventional dried petiole analysis to compare differences in the two methods and to determine if current calibration data could be used for interpreting results from sap meters. Both methods were found to he responsive to fertilizer treatments, and both showed declines in N and K as the season progressed. In early season measurements, dried petiole extracts were 3 to 10 times more concentrated than sap extracts, but the increase in slope with increasing N applied was remarkably similar. The two methods were strongly correlated (r=0.88) for the first two sampling periods, but we noted a change in the slope of the correlations with increasing time after first bloom. In essence, the change of slope indicated that the concentrations in the sap extracts were increasing relative to those in the dried extracts as the cotton plants became older. This corresponds with increasing difficulty in extracting sap samples from plants as they approach peak bloom. These preliminary results suggest that stage of growth, and by inference, moisture content, strongly affect the measured concentrations in the petiole sap, and will complicate the interpretation of results from this method. The meters could still be useful for early season measurements and for direct comparisons of "good" and "bad" samples within a field.